In a widely known active matrix type image display device, each pixel has a TFT (thin film transistor) made of amorphous Si, polycrystalline Si and the like, and a current control light emitting element such as an OLED (organic light emitting diode). In this image display device, different current values can be set for each pixel in response to image data to be displayed, thereby changing brightness level of each pixel.
A current Ids flowing between the source and the drain of a TFT when a light emitting element is caused to emit light is in proportion to the square of the potential difference between the source and the gate with respect to a source potential, namely the difference between a gate voltage Vgs and a threshold voltage Vth specified to the TFT. The brightness level of an OLED is substantially in proportion to the current density of a current flowing in the OLED, namely the current Ids.
A conventional image display device is disclosed for example in patent document (Japanese Patent Application Laid-Open No. 2006-309258), non-patent document (S. Ono et al., Proceedings of IDW '03, 255 (2003)), and others.
In the image display device discussed above, a plurality of pixel circuits having respective light emitting elements are arranged in a matrix. Further, a power source line is connected in common to pixel circuits for each row, or is connected in common to all pixel circuits. The power source line has wiring resistance. Thus, a potential applied from the power source line to each pixel circuit may change based on the wiring resistance and a flowing current. As a result, in a structure in which a plurality of pixel circuits are connected in common to an IC for supplying a negative power source potential through one power source line, for example, the power source potential to be supplied becomes higher (the absolute value of the negative potential becomes smaller) as a distance from the IC to a pixel circuit becomes longer. In a structure in which a plurality of pixel circuits are connected in common on a row-to-row basis to an IC for supplying a negative power source potential through one power source line, for example, the power source potential to be supplied becomes higher (the absolute value of the negative potential becomes smaller) as a distance from the IC to a pixel circuit becomes longer.
A source potential applied to a TFT, namely an NMOS transistor becomes higher in a pixel circuit as a power source potential to be supplied becomes higher, thereby lowering the gate voltage Vgs while reducing the current Ids flowing between the drain and the source. Namely, in a plurality of pixel circuits, when a power source potential to be supplied varies among the pixel circuits, the brightness level of an OLED which is in proportion to the current density of the current Ids also varies. This may generate trouble of irregularity in brightness level in an image to be displayed, so deterioration of image quality may occur. In a plurality of pixel circuits connected in common in the same row, when a power source potential to be supplied varies among the pixel circuits, the brightness level of an OLED which is in proportion to the current density of the current Ids also varies. This may generate trouble of irregularity in brightness level, cross talk or the like in an image to be displayed, so deterioration of image quality may occur.